102 
NATURE 
[| Fune 4, 1885 
powerful to enable the pictures on the cover to be clearly seen. 
By throwing the light upon a quickly rotating disk of zinc with 
a strip of white paper pasted across it, I convinced myself that 
the phenomenon was a subjective one: the successive images of 
the white strip always occurred in exactly the same position. 
I think these experiments go far towards explaining a matter 
which must have occurred as a difficulty to many besides my- 
self. Why is it that the illumination produced by a brilliant 
flash of lightning invariably appears to be of a quivering or 
intermittent character? We know that the actual duration of a 
single discharge is sensibly infinitesimal, and both reasoning and 
laboratory experiments would lead us to believe that a rapid 
series of discharges between two insulated bodies must at all 
events be of infrequent occurrence, and due to exceptional 
circumstances. Yet the quiver of the lightning-flash is pro- 
verbial. It would be interesting to ascertain by means of a 
revolving disk with a single white band across it, whether this is 
not in most cases a purely subjective phenomenon, due to a 
succession of after-images. SHELFORD BIDWELL 
Wandsworth, June 1 
Iridescent Crystals of Potassium Chlorate 
WITH regard to the above crystals, described by Prof. Stokes 
in NATURE (April 16, p. 565), I should like to suggest, with 
some diffidence, that the colours may be due, not to a continuous 
hemitropic crystal-film, but to a series of fine tubular cavities 
ranged parallel to each other between the two main portions of 
the crystal, such as not unfrequently occur on a large scale in 
Iceland spar, and appear to be due to bad fitting (so to speak) 
of hemitrope-films on the rest of the crystal (see Groth’s ‘* Phy- 
sikalische Krystallographie,” p. 441). 
The surfaces of these prismatic cavities, which may be of 
almost wave-length fineness, would form a series of furrows from 
which light would be reflected under the same conditions as from 
grooved surfaces like those of mother-of-pearl ; and, according 
to the usual laws of interference, we should expect such pheno- 
mena to occur as are described by Prof. Stokes: eg., non- 
polarisation of the light, predominance of rays of a particular 
refrangibility in the reflected beam, and total disappearance of 
this beam at two azimuths differing by 180°, when the length- 
dimension of the tubes lies parallel to the plane of incidence. 
I may mention that plates of opal—at any rate those portions 
which give a uniform colour—afford spectra extremely similar in 
character to those of the iridescent crystals: definite bright 
bands in the reflected light, and dark complementary bands in 
the transmitted light, changing their position in the spectrum 
with change of incidence. Now the iridescence of opal is pretty 
well known to be due to the reflection of light from the surfaces 
of rows of filaments imbedded in the mass (see Sir David 
Brewster’s paper in Brit. Ass. Reports, 1844, part 2, p. 9). 
The above hypothesis would also account for Prof. Stokes’s 
observation that the iridescent crystals were best formed when 
the solution was gently stirred; the molecules then, like a 
harassed army, being too much disturbed to ‘‘ fall in” as they 
should do. i 
I cannot say that I have yet succeeded in actually bringing 
out such rows of tubes under the microscope. It is easy to see 
with a $-inch power, when the illuminating beam is properly 
adjusted, a sort of wavy structure at the twin-film (like that seen 
in opal under the same conditions) ; but I have not yet made 
out such definite rows of cavities as would seem to be required 
to produce the singularly uniform sheet of colour. They may 
be beyond the power of a microscope altogether. 
Eton College H. G. MADAN 
P.S.—Since I wrote the above, Prof. Stokes has very kindly 
pointed out that opal spectra are fully described by Mr. Crookes 
in Proc. Koy. Soc., xvii. 448. One opal micro-section which I 
have gives a spectrum almost exactly like No. 12 in that paper, 
but the band is slightly less refrangible than the D line at an 
incidence of 20°. 
Pre-Existence and Post-Existence of Thought 
fo express any views on these subjects one might well have 
hesitated some years ago, as hereditary transmission, tolerated as 
a doctrine, chiefly with regard to the breeding of sheep and 
dogs, was held to be unphilosophical. Darwin has settled the 
matter in the domain of science, but perhaps without altogether 
disposing of prejudices. Mr. Galton and myself have long since 
dealt with less material aspects of heredity. 
What I want to bring on the scene of consideration is the 
common notion of throwing the great-grandsire. Few now are 
inclined to doubt about this. 
Throwing the grandsire in the case of man may be taken to 
represent a period of a hundred years of transmission until birth 
for three generations, and consequently such period of the pre- 
existence of a thought or habit, which is in one’s self, and of 
which there may be an actual register. It is better to select the 
example of a man, and of a great-grandsire, rather than of a 
father, because it carries with it a period of remoteness. 
Taking then a period of a century for pre-existence in the 
past, there comes the consideration of vitality of occurrence 
in the future, as we know of it from the past. Taking a 
generation for the birth of offspring, and adding to it three 
generations for a great-grandchild, we have, say, one hundred 
and fifty years. Adding together the two periods, we obtain an 
epoch of transmission in pre-existence and of possible existence 
of two hundred and fifty years. 
One such epoch antecedently and another subsequently may 
be counted in round numbers five hundred years, and we shall 
see our way to a thousand. 
Perhaps after all we know less in detail of the transmission of 
osseous peculiarities than we assume. There are the means in 
some cases of examining bony structures of epochs and intervals 
of 1,000, 2,000, 5,000, 10,000 and more years, and we can trace 
forms of development. What we further want is the power of 
witnessing the successive individual variations, which is attended 
with diversity from other beings, and constitutes identity. 
Though identity may to some extent repose in the process of a 
bone, in the way in which a vein or sinew has traversed, we 
have evidence of this rather collectively in a species or a variety, 
than in the mode in which it periodically or irregularly influences 
a series of individuals. 
The transmission of physical peculiarities in the soft tissues is © 
equally permanent, and can be better traced in the individual. 
There are good examples of it in well-known cases of family 
features, and of racial types. 
We are now familiar with cases of the hereditary transmission 
of mental qualities, as in that of the Bernouilli family, though 
perhaps one of the most remarkable instances is that of the 
Ottoman dynasty, members of which for several hundred years 
have displayed capacity, and yet here we have only the male 
elements of transmission, as in the marriages there has been a 
great mixture of races. It may, however, be that the Turk, 
the so-called Circassian (sometimes a Daghestani), and the 
Georgian are more nearly allied than we have been disposed to 
consider. 
The problem now before us may be treated, irrespective of 
what laws there may be of male or female transmission, as in 
breeding, the qualities of an ancestor on either side and of 
either sex may be reproduced, and it is this reproduction and 
transmission of mental qualities which is to be considered. 
The mental qualities must be distinguished from the osseous 
structure or the soft tissues in degree. There is a vast differ- 
ence in extent, and probably in distinction, between the trans- 
mission of some feature and that of conscious or unconscious 
thought or even of a dream. 
My own term of ‘‘ unconscious thought ” I prefer to ‘‘ uncon- 
scious cerebration,” because the main distinction between 
conscicus and unconscious thought is chiefly, if not wholly, 
dependent on the quality of consciousness. One reason for 
retaining it here is with reference to dreams. Whatever may be 
the operation of recording other thoughts, dreams are certainly 
preserved during life as effectually as any kind of thought, 
although no organ of seeing or hearing is concerned. 
If it is remarkable that purely physical properties should be 
preserved in the germ of a minute animal, it is much more so 
that any process allied to the operations of thought should be 
preserved—the influence of events, the influence of dreams, used 
age after age to constitute the mind of this day. It will be per- 
ceived that I am speaking very loosely and vaguely, to some 
extent of set purpose, to bring under consideration a general 
question of long time in the transmission of a mental process, 
whether connected with what is called instinct or unconscious 
thought, and without limiting the discussion more than is 
necessary with postulates or set definitions. 
HybrE CLARKE 
32, St. George’s Square, S.W., May 23 
